Under both natural and laboratory conditions, female rats in estrus exhibit spontaneous and predictable patterns of sexual contacts with males which appear to regulate or pace the temporal characteristics of copulatory stimuli received. Two consequences of the females' pacing of copulatory stimulation are an abbreviation of the length of the period of estrus and acute increases in serum concentrations of the 5a-reduced androgen, 3a-androstanediol. These data suggest that this androgen, released from the ovary in response to temporally-appropriate coital stimuli, contributes to decreasing sexual responsiveness at the end of estrus. The experiments proposed in this application will define the characteristics of the behavioral stimuli which contribute to estrus abbreviation and 3a-androstanediol release, determine whether increases in circulating 3a-androstanediol play a physiological role in estrus abbreviation, and determine whether increased levels of 3a-androstanediol are a result of pituitary hormone release. These studies will examine the reciprocal relationships between the endocrine system and sexual responsiveness in the female rat -- which relationships may serve to regulate reproductive processes insuring species survival. In addition, these studies will elucidate whether 5a-reduced androgens, known to have strong inhibitory effects on sexual receptivity in ovariectomized hormone-primed rats, play a role under physiological conditions in modulating the display of sexual behavior in the intact female rat.